The present invention relates to electric-to-pressure transducers, and particularly to electric-to-pressure transducers employing a feedback loop to control fluid pressure.
Electric-to-pressure transducers convert an electric signal to a pressure signal which may be used for control of remote pneumatic or hydraulic devices such as pilot-actuated valves. Such transducers are commercially available in many forms, one of which is a current-to-pressure transducer which converts an input signal of 4-20 milliamperes (ma) DC into a proportional pneumatic gauge pressure of 3-15 pounds per square inch (psig). This type of transducer is available from Johnson Controls (Model N-6810), Fairchild Industrial Products Co., Winston-Salem, N.C. (Model T5000-04), and Barber Colman (Model CP-8551 and CP-8561) as an open-loop device. Closed-loop control is possible with the Robertshaw Model 2899-040 transducer, a motorized transducer with a potentiometer coupled to the motor shaft for indicating shaft position to an external controller.
U.S. Pat. No. 4,481,967 to Frick discloses a two-wire 4-20 ma current-to-pressure converter utilizing pressure feedback. Capacitive and piezoresistive pressure transducers are disclosed, and linear control is achieved using a servo valve. While a servo valve enables linear pressure control, it is an expensive device, particularly in comparison to a standard on-off valve such as a solenoid valve.
Solenoid valves are widely viewed as unsuitable for precision applications, particularly for any application requiring linear control, because the magnetic field within a solenoid produces a continuous mechanical force on the solenoid plunger which tends to drive the plunger to an extreme end position. Open-loop pressure control is further impractical with a standard on-off valve because electrical control of valve element position is not repeatable in many working environments due to external factors such as vibration.
A solenoid valve is used in the closed-loop pressure regulator of U.S. Pat. No. 4,253,480 to Kessel et al. for producing a continuously variable opening, but a relatively complex pulse-width modulating scheme is employed to achieve the desired variations in valve position.
Existing electric-to-pressure transducers such as those described above have one or more disadvantages including inaccuracy, nonlinearity, complex mechanical construction or electrical circuitry, high cost, excessive size, and sensitivity to vibration or other environmental conditions.